A May 31 tornado near El Reno, OK, that killed eight people was preliminarily rated an EF-3 on the Enhanced Fujita scale. After reviewing data from a research radar near the storm, the preliminary rating was upgraded to an EF-5. This was a controversial move at the time, with many arguing tornadoes should be rated solely on the damage they do while others arguing all available data should be taken into account.
Late Friday, the NWS officially rated the tornado an EF-3, not an EF-5, citing the fact that EF-3 damage was found. Local media meteorologists and others were notified via phone calls, but no press release or public information statement was provided at that time.
Keli Pirtle from NOAA Public Affairs in Norman provided me this statement via email, explaining the reason behind the change:
NOAA National Weather Service officially rated the El Reno, Okla., tornado of May 31, 2013, an EF3 on the Enhanced Fujita Scale. It will be listed with that rating in Storm Data, a publication considered the official record of weather events.
WFO Norman announced a preliminary rating of EF5 on June 4, 2013, following analysis of velocity data taken by two high-resolution mobile research radars. This analysis showed wind speeds of at least 295 miles an hour within 500 feet of the ground. The threshold for an EF5 designation is 200 miles an hour.
Despite the radar-measured wind speeds, the survey team did not find damage that would support a rating higher than EF3.
While the wind measurements from the mobile radars are considered reliable, NWS policy for determining EF ratings is based on surveys of ground damage.
NWS is exploring whether policy should change to allow the use of experimental radar data in future EF determinations.